Solid alkaline fuel cell

ABSTRACT

The solid alkaline fuel cell has a cathode that is supplied with an oxidant which contains oxygen, an anode that is supplied with a fuel which contains hydrogen atoms, and an inorganic solid electrolyte that is disposed between the anode and the cathode and that exhibits hydroxide ion conductivity. The inorganic solid electrolyte enables permeation of a fuel in an amount that produces carbon dioxide at the cathode of greater than or equal to 0.04 μmol/s·cm2 and less than or equal to 2.5 μmol/s·cm2 per unit surface area of a cathode-side surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2018/046385,filed Dec. 17, 2018, which claims priority to Japanese Application No.2017-241272, filed Dec. 18, 2017, the entire contents all of which areincorporated hereby by reference.

TECHNICAL FIELD

The present invention relates to a solid alkaline fuel cell.

BACKGROUND ART

Alkaline fuel cells (AFC) are known as a fuel cell that operates at arelatively low temperature (for example, less than or equal to 250degrees C.).

Various liquid fuels or gaseous fuels can be used in an AFC and forexample, use of methanol as a fuel causes the following electrochemicalreaction.

CH₃OH+6OH⁻→6e ⁻+CO₂+5H₂O  Anode:

3/2O₂+3H₂O+6e ⁻→6OH⁻  Cathode:

CH₃OH+3/2O₂→CO₂+2H₂O  Overall:

In this context, Japanese Patent Application Laid-Open No. 2016-071948proposes a solid alkaline fuel cell in which an inorganic solidelectrolyte that exhibits hydroxide ion conductivity is configured as alayered double hydroxide (LDH) that does not exhibit liquid permeabilityand gas permeability.

LDH is expressed by the general formula [M²⁺ _(1-x)M³⁺ _(x)(OH)₂][A^(n−)_(x/n).mH₂O] (wherein M²⁺ is a divalent cation, M³⁺ is a trivalentcation, and A^(n−) is an n-valent anion).

SUMMARY OF INVENTION

As shown by the above reaction formula, a solid alkaline fuel cellconsumes both oxygen (O₂) and water (H₂O) at the cathode, and thereforethe cathode must be supplied with an oxidant that includes oxygen andwater (for example, humidified air).

However, supplying the cathode with an oxidant that contains waterrequires not only equipment for humidifying the oxidant (for example, ahumidifier, a water tank or the like) but also consumes the energy forhumidifying the oxidant.

For this reason, there is a need for a new approach for efficient supplyof water to the cathode.

The present invention has the object of providing a solid alkaline fuelcell that enables efficient supply of water to the cathode.

The solid alkaline fuel cell according to the present inventioncomprises a cathode that is supplied with an oxidant which containsoxygen, an anode that is supplied with a fuel which contains hydrogenatoms, and an inorganic solid electrolyte that is disposed between theanode and the cathode and that exhibits hydroxide ion conductivity. Theinorganic solid electrolyte enables permeation of a fuel in an amountthat produces carbon dioxide at the cathode of greater than or equal to0.04 μmol/s·cm² and less than or equal to 2.5 μmol/s·cm² per unitsurface area of a cathode-side surface.

The present invention provides a solid alkaline fuel cell that enablesefficient supply of water to the cathode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view illustrating a schematic configurationof a solid alkaline fuel cell.

FIG. 2 is an exploded perspective view illustrating a solid alkalinefuel cell according to an Example.

DESCRIPTION OF EMBODIMENTS

Solid Alkaline Fuel Cell 10

A solid alkaline fuel cell 10 is a type of alkaline fuel cell (AFC) thatoperates at a relatively low temperature. The operation temperature ofthe solid alkaline fuel cell 10 according to the present embodiment is50 degrees C. to 250 degrees C. The solid alkaline fuel cell 10 operatesfor example with methanol and generates power with the electrochemicalreaction below.

3/2O₂+3H₂O+6e ⁻→6OH⁻  Cathode 12:

CH₃OH+6OH⁻→6e ⁻+CO₂+5H₂O  Anode 14:

CH₃OH+3/2O₂→CO₂+2H₂O  Overall:

FIG. 1 is a cross sectional view illustrating a schematic configurationof the solid alkaline fuel cell 10. The solid alkaline fuel cell 10includes a cathode 12, an anode 14, and an inorganic solid electrolyte16.

The cathode 12 is a positive electrode that is generally termed the airelectrode. During power generation by the solid alkaline fuel cell 10,an oxidant supply means 13 supplies the cathode 12 with an oxidant thatincludes oxygen (O₂). The oxidant may include use of air.

In this context, the inorganic solid electrolyte 16 according to thepresent embodiment is configured to enable permeation (cross-over)towards the cathode 12 of a portion of a fuel that is supplied to theanode 14. The cathode 12 produces carbon dioxide (CO₂) and water (H₂O)by a reaction between the fuel that has permeated through the inorganicsolid electrolyte 16 to the cathode 12 and oxygen that is contained inthe oxidant. For example, when methanol is used as a fuel, the followingreaction occurs at the cathode 12.

CH₃OH+3/2O₂→CO₂+2H₂O  Cathode 12:

Water that is produced from the fuel that has permeated the inorganicsolid electrolyte 16 to the cathode 12 is used in the electrochemicalreaction at the cathode 12 as described above.

When the entire amount of water that is required for the electrochemicalreaction at the cathode 12 is produced from the fuel, the oxidant thatis supplied to the cathode 12 may substantially omit a water content. Inthis configuration, there is no requirement to provide equipment (forexample, a humidifier, water tank, or the like) to humidify the oxidant,and furthermore, there is no energy consumed in humidifying the oxidant.

On the other hand, when only a portion of the water that is required forthe electrochemical reaction at the cathode 12 is produced from thefuel, the oxidant that is supplied to the cathode 12 preferably includeswater. In this configuration, although there is a requirement to provideequipment for humidifying the oxidant, the equipment may be downscaledto thereby reduce the energy consumed in humidifying the oxidant. It isnoted that humidified air is suitable as an oxidant that includes oxygenand water.

The term “water (H₂O)” as used in the specification may denote watervapor in a gaseous state, moisture in a liquid state, and a gas-liquidmixture of water vapor and moisture.

The cathode 12 may be configured to include a known cathode catalystthat is used in an alkaline fuel cell, but there is no limitation inthis regard. An example of a cathode catalyst includes a group 8 to 10element (an element that belongs to groups 8 to 10 in the IUPAC formatperiodic table) such as a platinum group element (Ru, Rh, Pd, Os, Ir,Pt), an iron group element (Fe, Co, Ni), or the like, a group 11 element(an element that belongs to group 11 in the IUPAC format periodic table)such as Cu, Ag, Au, or the like, rhodium phthalocyanine,tetraphenylporphyrin, Co salen, Ni salen (salen=N,N′-bis (salicylidene)ethylenediamine), silver nitrate, and arbitrary combinations thereof.Although there is no particular limitation in relation to the amount ofsupported catalyst in the cathode 12, a value of 0.1 to 10 mg/cm² ispreferred, and 0.1 to 5 mg/cm² being more preferred. It is preferredthat the cathode catalyst is supported on carbon. A preferred example ofthe cathode 12 or a catalyst that used to configure the cathode 12includes platinum-supporting carbon (Pt/C), paladium-supporting carbon(Pd/C), rhodium-supporting carbon (Rh/C), nickel-supporting carbon(Ni/C), copper-supporting carbon (Cu/C) and silver-supporting carbon(Ag/C).

Although there is no particular limitation in relation to the method ofpreparation for the cathode 12, for example, it may be formed by mixinga cathode catalyst with a support and a binder as desired to form apaste, and coating the paste mixture onto one surface of the inorganicsolid electrolyte 16.

The anode 14 is a negative electrode that is generally termed the fuelelectrode. During power generation by the solid alkaline fuel cell 10, afuel supply means 15 supplies the anode 14 with a fuel that includeshydrogen atoms (H).

A fuel that contains hydrogen atoms can react with hydroxide ions (OH—)at the anode 14, and may enable production of water by reacting withoxygen at the cathode 12.

This type of fuel may be configured as either a liquid fuel or gaseousfuel. A liquid fuel may be a liquid of a fuel compound itself, or may bea solid fuel compound that has been dissolved in a liquid such as water,alcohol, or the like.

For example, the fuel compound includes (i) types of hydrazine such ashydrazine (NH₂NH₂), hydrated hydrazine (NH₂NH₂.H₂O), hydrazine carbonate((NH₂NH₂)₂CO₂), hydrazine sulfate (NH₂NH₂.H₂SO₄), monomethyl hydrazine(CH₃NHNH₂), dimethyl hydrazine ((CH₃)₂NNH₂, CH₃NHNHCH₃) and carboxylichydrazide ((NHNH₂)₂CO), or the like, (ii) urea (NH₂CONH₂), (iii) ammonia(NH₃), (iv) heterocyclic compounds such as imidazole, 1,3,5-triazine,3-amino-1,2,4-triazole, or the like, and (v) hydroxylamines such ashydroxylamine (NH₂OH), hydroxylamine sulfate (NH₂OH.H₂SO₄), or the like,and combinations thereof.

Those compounds of the above fuel compounds that do not contain carbon(that is to say, hydrazine, hydrated hydrazine, hydrazine sulfate,ammonia, hydroxylamine, hydroxylamine sulfate, or the like) not onlyenable enhanced durability by avoiding any problems associated withcatalyst poisoning by carbon monoxide, but also avoid carbon dioxideemissions.

The fuel compound may be used without modification as a fuel or may beused as a solution by dissolving in water and/or alcohol (for example, alower alcohol such as methanol, ethanol, propanol, isopropanol, or thelike). For example, since fuel compounds such as hydrazine, hydratedhydrazine, monomethyl hydrazine and dimethyl hydrazine are in a liquidstate, use is possible as a liquid fuel without modification.Furthermore, solids such as hydrazine carbonate, hydrazine sulfate,carboxylic hydrazide, urea, imidazole, 3-amino-1,2,4-triazole andhydroxylamine sulfate can be dissolved in water. Solids such as1,3,5-triazine and hydroxylamine can be dissolved in alcohol. A gas suchas ammonia can be dissolved in water. In this manner, solid fuelcompounds can be used as a liquid fuel by dissolution in water oralcohol. When using a fuel compound by dissolution in water and/oralcohol, the concentration of the fuel compound in solution for exampleis 1 to 90 wt % and is preferably 1 to 30 wt %.

Furthermore, hydrocarbon liquid fuels that include types of ethers ortypes of alcohol such as methanol, ethanol, or the like, hydrocarbongases such as methane or the like, or pure hydrogen or the like may beused as a fuel without modification. In particular, methanol is suitableas a fuel for use in the solid alkaline fuel cell 10 according to thepresent embodiment. Methanol may be in gaseous state, liquid state or asa gas-liquid mixture.

Although the anode 14 may be configured to include a known anodecatalyst that is used in an alkaline fuel cell, there is no particularlimitation in this regard. An example of an anode catalyst includes ametal catalyst such as Pt, Ni, Co, Fe, Ru, Sn and Pd or the like.Although it is preferred that a metal catalyst is supported on a supportsuch as carbon or the like, it may be configured as an organometalcomplex using the metal atoms of the metal catalyst as the centralmetal, or supported by use of such an organometal complex as a support.Furthermore, a diffusion layer that is configured using a porousmaterial or the like may be disposed on a surface of the anode catalyst.Preferred examples of the anode 14 or the catalyst that is used toconfigure the anode include nickel, cobalt, silver, platinum supportingcarbon (Pt/C), palladium supporting carbon (Pd/C), rhodium supportingcarbon (Rh/C), nickel supporting carbon (Ni/C), copper supporting carbon(Cu/C) and silver supporting carbon (Ag/C).

Although there is no particular limitation in relation to the method ofpreparation for the anode 14, for example, it may be formed by mixing ananode catalyst with a support and a binder as desired to form a paste,and coating the paste mixture onto the surface of the inorganic solidelectrolyte 16 that is opposite to the cathode 12.

The inorganic solid electrolyte 16 is disposed between the cathode 12and the anode 14. The inorganic solid electrolyte 16 includes acathode-side surface 16S and an anode-side surface 16T. The cathode-sidesurface 16S is a region in the outer surface of the inorganic solidelectrolyte 16 that is exposed in a space in which the cathode 12 isdisposed, and faces the cathode 12. The anode-side surface 16T is aregion in the outer surface of the inorganic solid electrolyte 16 thatis exposed in a space in which the anode 14 is disposed, and faces theanode 14.

The inorganic solid electrolyte 16 is a ceramic that exhibits hydroxideion conductivity. The higher the hydroxide ion conductivity in theinorganic solid electrolyte 16, the more such a configuration ispreferred and it is typically configured as 10⁻⁴ to 10⁻¹ S/m.

The inorganic solid electrolyte 16 may be configured as a layered doublehydroxide (referred to below as “LDH”). In such a configuration, theinorganic solid electrolyte 16 exhibits superior heat resistance anddurability when compared with a configuration in which an organicmaterial such as AEM (anion exchange membrane) is used as anelectrolyte.

LDH has a basic composition that is expressed by the general formula of[M²⁺ _(1-x)M³⁺ _(x)(OH)₂][A^(n−) _(x/n).mH₂O] (wherein M²⁺ is a divalentcation, M³⁺ is a trivalent cation, and A^(n−) is an n-valent anion, n isan integer that is greater than or equal to 1, and x takes a value of0.1 to 0.4).

In the general formula above, although M²⁺ may be an arbitrary divalentcation, it preferably includes Mg²⁺, Ca²⁺, and Zn²⁺, and more preferablyis Mg²⁺. M³⁺ may be an arbitrary trivalent cation, preferably includesAl³⁺ or Cr³⁺, and more preferably is Al³⁺. A^(n−) is an arbitrary anion,preferably includes OH⁻ and CO₃ ²⁻.

Therefore in the general formula above, it is particularly preferredthat M²⁺ includes Mg²⁺, M³⁺ includes Al³⁺ and A^(n−) includes OH⁻ andCO₃ ²⁻. Although n is an integer that is greater than or equal to 1, itpreferably takes a value of 1 or 2. Although x takes a value of 0.1 to0.4, it preferably takes a value of 0.2 to 0.35. An arbitrary realnumber is denoted by m.

Furthermore, a portion or all of M³⁺ in the general formula above may besubstituted by cations having a valency of 4 or more, and in thatconfiguration, the coefficient x/n of the anion A^(n−) in the generalformula may be suitably varied.

The inorganic solid electrolyte 16 in the present embodiment isconfigured to enable permeation of a portion of the fuel supplied to theanode 14 to the cathode 12 side. As described above, the cathode 12produces carbon dioxide and water using a reaction of fuel that haspermeated through the inorganic solid electrolyte 16 from the anode 14to the cathode 12 with oxygen that is contained in the oxidant.Therefore the fuel permeation characteristics of the inorganic solidelectrolyte 16 (that is to say, the degree to which fuel is allowed topermeate) can be defined with reference to the carbon dioxide amountthat is produced at the cathode 12. The carbon dioxide amount that isproduced at the cathode 12 may be defined as the produced amount ofcarbon dioxide per unit surface area of the cathode-side surface 16S.

More specifically, the inorganic solid electrolyte 16 enables permeationof an amount of fuel that generates an amount of carbon dioxide that isgreater than or equal to 0.04 μmol/s·cm² and less than or equal to 2.5μmol/s·cm² per unit surface area of the cathode-side surface 16S.

Since a configuration in which the produced amount of carbon dioxide perunit surface area is greater than or equal to 0.04 μmol/s·cm² suppliesthe cathode 12 with water produced from the fuel that has permeated theinorganic solid electrolyte 16, when generating power at a rated load(0.3 A/cm², air utilization rate Ua 50%), the output of the solidalkaline fuel cell 10 can be maintained even when the output of thehumidifier is reduced. Furthermore, a configuration in which theproduced amount of carbon dioxide per unit surface area is less than orequal to 2.5 μmol/s·cm² inhibits an adverse effect on thecharacteristics of the inorganic solid electrolyte 16 and maintains theoutput of the solid alkaline fuel cell 10 as well as suppressing therate of increase in the fuel supply amount that is required due to fuelpermeating to the cathode 12.

In addition, a configuration in which the produced amount of carbondioxide per unit surface area is greater than or equal to 0.15μmol/s·cm², or furthermore a configuration in which the produced amountof carbon dioxide per unit surface area is greater than or equal to 0.6μmol/s·cm² maintains the output of the solid alkaline fuel cell 10 whengenerating power with reference to a rated load even when there is afurther reduction in the output of the humidifier.

Furthermore, a configuration in which the produced amount of carbondioxide per unit surface area is less than or equal to 1.7 μmol/s·cm²enables additional suppression of the increase rate in the fuel supplyamount and further inhibition of an adverse effect on thecharacteristics of the inorganic solid electrolyte 16 itself.

The carbon dioxide amount produced from the fuel at the cathode 12 isobtained by using gas chromatograph equipment to measure the carbondioxide amount [μmol] contained in the oxidant that is totally recoveredafter passing through the cathode 12. However when carbon dioxide iscontained in the oxidant itself that is supplied from the oxidant supplymeans 13, the carbon dioxide amount contained in the oxidant itself issubtracted from the measured carbon dioxide amount to thereby enableclear comprehension of the carbon dioxide amount produced from the fuel.In addition, the produced amount of carbon dioxide per unit surface areaof the cathode-side surface 16S may be calculated by dividing the carbondioxide amount produced from the fuel by the surface area of thecathode-side surface 16S and the operation time.

A fuel permeation function may be imparted to the inorganic solidelectrolyte 16 for example by adopting any of a first method, a secondmethod, or a combination of the first method and second method asdescribed below.

The first method of imparting a fuel permeation function is a method ofproviding through holes in the inorganic solid electrolyte 16. Thethrough holes are produced by piercing an inner portion of the inorganicsolid electrolyte 16 from the anode-side surface 16T to the cathode-sidesurface 16S. The inner diameter and number of through holes may besuitably set to thereby produce carbon dioxide at the cathode 12 that isgreater than or equal to 0.04 μmol/s·cm² and less than or equal to 2.5μmol/s·cm² per unit surface area of the cathode-side surface 16S. Thethrough holes are preferably disposed in proximity to the supply port ofthe oxidant supply means 13 in the inorganic solid electrolyte 16. Inthis manner, water that is produced by reaction with oxygen of the fuelthat flows from the through holes to the cathode 12 can be transferredwith the oxidant to the whole cathode 12.

The second method of imparting a fuel permeation function is a method ofproviding pores in the inorganic solid electrolyte 16. It is preferredthat the pores are formed to connect an inner portion of the inorganicsolid electrolyte 16 from the anode-side surface 16T to the cathode-sidesurface 16S. There is no particular limitation on the shape of thepores, and they may be configured with an irregular shape, or as a meshor the like. The pores may be formed throughout the inorganic solidelectrolyte 16, or may be formed only in a portion of the inorganicsolid electrolyte 16. The inner diameter, length and number of the poresmay be suitably set to thereby produce carbon dioxide at the cathode 12that is greater than or equal to 0.04 μmol/s·cm² and less than or equalto 2.5 μmol/s·cm² per unit surface area of the cathode-side surface 16S.

However, when a substrate as described below is included in theinorganic solid electrolyte 16, the second method (method of provisionof pores) can only be used when the substrate is configured using amaterial that can withstand the firing temperature (for example, greaterthan or equal to 400 degrees C.). The method of manufacturing theinorganic solid electrolyte 16 will be described below.

Although the inorganic solid electrolyte 16 may be configured only byuse of a particle group that contains an inorganic solid electrolyticsubstance that exhibits hydroxide ion conductivity, an additivecomponent to assist in the density or hardness of that particle groupmay also be included.

The inorganic solid electrolyte 16 may be configured as a complex usinga substrate formed as a porous body that exhibits an open poreconfiguration and an inorganic solid electrolytic substance (forexample, LDH) that is precipitated and grown into the pores to therebyfill the pores of the porous body. The porous body may be configured bya ceramic material such as alumina, zirconia or the like or aninsulating material such as a porous sheet or the like that is formedfrom a foam resin or fibrous substance.

The inorganic solid electrolyte 16 may be configured in any of a plateshape, membrane shape or stacked shape. When the inorganic solidelectrolyte 16 has a membrane or stacked shape, the membrane or stackedshaped inorganic solid electrolytic substance of the inorganic solidelectrolyte 16 may be formed in the porous substrate or on the poroussubstrate. When the inorganic solid electrolyte 16 has a membrane orstacked shape, the thickness of the inorganic solid electrolyte 16 maybe configured as less than or equal to 100 μm, preferably less than orequal to 75 μm, more preferably less than or equal to 50 μm, still morepreferably less than or equal to 25 μm, and particularly more preferablyless than or equal to 5 μm. The resistance of the inorganic solidelectrolyte 16 decreases as the thickness of the inorganic solidelectrolyte 16 is reduced. The lower limiting value of the thickness ofthe inorganic solid electrolyte 16 may be set with reference to a givenuse, but is preferably greater than or equal to 1 μm and more preferablygreater than or equal to 2 μm in order to maintain a certain degree ofrigidity. When the inorganic solid electrolyte 16 has a plate shape, thethickness of the inorganic solid electrolyte 16 may be configured as0.01 to 0.5 mm, preferably 0.02 to 0.2 mm, and more preferably 0.05 to0.1 mm.

Method of Manufacturing Inorganic Solid Electrolyte 16

An example of a method of manufacturing an inorganic solid electrolyte16 will be described. The method of manufacture in the followingdescription is performed by molding and firing an LDH powder of LDH thatis typically hydrotalcite to thereby form an oxide fired body, and thenregenerating the fired body to LDH and removing excess moisture. Themethod of manufacture enables simple and stable manufacture of a denseinorganic solid electrolyte 16.

1. Preparation of LDH Powder

An LDH powder is prepared that has the basic composition expressed bythe general formula above: [M²⁺ _(1-x)M³⁺ _(x)(OH)₂][A^(n−) _(x/n).mH₂O](wherein M²⁺ is a divalent cation, M³⁺ is a trivalent cation, and A^(n−)is an n-valent anion, n is an integer greater than or equal to 1, and xis 0.1 to 0.4). This type of LDH powder is commercially available, or isa starting material that is prepared by use of a known method such as aliquid phase synthesis method or the like that uses sulfate salts orchlorides.

Although there is no particular limitation on the particle diameter ofthe LDH powder, a volume reference D50 average particle diameter ispreferably 0.1 μm to 1.0 μm, and more preferably 0.3 μm to 0.8 μm. Whenthe particle diameter of the LDH powder is excessively fine, the powderaggregates and tends to cause residual pores during molding. Anexcessively large particle diameter of the LDH powder has an adverseeffect on molding characteristics.

The LDH powder may be calcined into an oxide powder. The calcinetemperature in that case may be set within a temperature range that doesnot greatly change the starting material particle diameter and, forexample, is preferably less than or equal to 500 degrees C., and morepreferably 380 degrees C. to 460 degrees C.

2. Preparation of Green Body

Next, the LDH powder is molded to form a green body. The molding step ispreferably performed by pressure molding for example so that therelative density of the green body is 43% to 65%, and more preferably45% to 60%, and still more preferably 47% to 58%. The pressure moldingmay be performed by use of a known method such as a mold uniaxial press,cold isostatic pressurization (CIP), slip casting, or extrusion molding,or the like. However, when the LDH powder is calcined to form an oxidepowder, there is a limitation to dry molding methods.

When the inorganic solid electrolyte 16 is imparted with a fuelpermeation function by use of the second method (method of provision ofpores) as described above, the green body is formed by addition of apore forming agent (for example, an acrylate polymer, methylcellulose orthe like) to the LDH powder. Pores are formed in an inner portion of theoxide fired body by firing and removal of the pore forming agent in afiring step as described below. The inner diameter, length and number ofpores may be adjusted by the amount and particle diameter of the poreforming agent.

The relative density of the green body is calculated by using atheoretical density to divide a density that is calculated withreference to the weight and dimensions of the green body. Since theweight of the green body is affected by adsorbed moisture, a uniquevalue is preferably obtained by measuring a green body that isconfigured from LDH powder that has been stored for greater than orequal to 24 hours at room temperature in a desiccator in a relativehumidity of less than or equal to 20%, or by measuring the relativedensity after storage of the green body under the above conditions.

However, when the LDH powder is calcined to form an oxide powder, therelative density of the green body is preferably 26% to 40% and morepreferably 29% to 36%. The relative density when using an oxide powderassumes that the respective metal elements that configure the LDH areconverted into their respective oxides by calcining, and is calculatedusing a denominator that is calculated as a converted density of amixture of each oxide.

3. Firing Step

Next, the green body is fired to form an oxide fired body. The firingstep is preferably performed so that the oxide fired body has a weightthat is 57% to 65% of the weight of the green body, and/or a volume thatis 70% to 76% of the volume of the green body.

A configuration in which the weight of the oxide fired body is greaterthan or equal to 57% of the weight of the green body inhibits theformation of heterogeneous phases that are not regenerated duringregeneration to LDH in subsequent steps. A configuration in which it isless than or equal to 65% of the weight of the green body enables theformation of a sufficient density in the following steps in response tosufficient firing. Furthermore a configuration in which the volume ofthe oxide fired body is greater than or equal to 70% of the volume ofthe green body inhibits the formation of heterogeneous phases that arenot regenerated during regeneration to LDH in subsequent steps, andinhibits the formation of cracks. A configuration when it is less thanor equal to 76% the volume of the green body enables the formation of asufficient density in the following steps in response to sufficientfiring.

However, when the LDH powder is calcined to form an oxide powder, it ispreferred that firing is performed so that the oxide fired body has 85%to 95% of the weight of the green body and/or has a volume that isgreater than or equal to 90% of the volume of the green body.

Also, irrespective of whether or not the LDH powder is calcined, therelative density of the oxide fired body is preferably configured to be20% to 40% and more preferably 20% to 35%, and still more preferably 20%to 30% with reference to an oxide conversion. A relative density withreference to an oxide conversion is a relative density that assumes thatthe respective metal elements that configure the LDH are converted intotheir respective oxides by firing, and is obtained with reference to adenominator that is calculated as a converted density of a mixture ofeach oxide.

The firing temperature for the green body may be configured as 400degrees C. to 850 degrees C., and is preferably 700 degrees C. to 800degrees C. The firing step preferably includes a step in which thefiring temperature above is maintained for greater than or equal to 1hour, and preferably 3 to 10 hours. Furthermore, the rate of temperatureincrease to reach a firing temperature is preferably less than or equalto 100 degrees C./h, more preferably 5 degrees C./h to 75 degrees C./h,and still more preferably 10 degrees C./h to 50 degrees C./h in order toprevent fracture of the green body that is caused by emission ofmoisture or carbon dioxide due to a sharp temperature increase.Therefore the total firing time from temperature increase to temperaturedecrease (less than or equal to 100 degrees C.) is preferably greaterthan or equal to 20 hours, more preferably 30 hours to 70 hours andstill more preferably 35 hours to 65 hours.

When a pore forming agent is added to the green body in the preparationstep for the green body as described above, pores are formed in an innerportion of the oxide fired body by firing and removal of the poreforming agent during the firing step.

4. Regeneration to LDH

Next, the oxide fired body is retained in or immediately above anaqueous solution that contains an n-valent anion (A^(n−)) as describedabove and is regenerated to LDH to thereby obtain an LDH solidified bodythat is moisture rich. That is to say, the LDH solidified body that ismanufactured by this method unavoidably contains excess moisture.

It is noted that the anions that are contained in the aqueous solutionmay be the same anions as the anions that are contained in the LDHpowder, or may be a different type of anion.

The retention of the oxide fired body in the aqueous solution orimmediately above the aqueous solution is preferably performed by amethod of hydrothermal synthesis in a sealed vessel. An example of asealed vessel includes a sealed vessel manufactured from Teflon(Registered Trademark). The outer side of the sealed vessel preferablyincludes provision of a jacket of stainless steel or the like.

LDH conversion is preferably performed by retaining the oxide fired bodyat greater than or equal to 20 degrees C. and less than 200 degrees C.,and in a state of contact of at least one surface of the oxide firedbody with the aqueous solution. A more preferred temperature is 50degrees C. to 180 degrees C. and a still more preferred temperature is100 degrees C. to 150 degrees C. The oxide fired body is preferablyretained for greater than or equal to 1 hour at this LDH conversiontemperature, more preferably retained at greater than or equal to 2hours, and still more preferably retained at greater than or equal to 5hours. In this manner, sufficient regeneration to LDH is promoted whileinhibiting residual heterogeneous phases. It is noted that although noproblems arise in the event that the retention time is excessively long,suitable settings are appropriate when efficiency is taken into account.

Ion exchanged water can be used when using carbon dioxide (carbonateions) in the air as a type of anion for the aqueous solution thatcontains n-valent anions that are used in regeneration to LDH. It isnoted that when performing hydrothermal synthesis in a sealed vessel,the oxide fired body may be immersed in the aqueous solution, or may betreated in a state in which a jig is used and at least one surface makescontact with the aqueous solution. When treatment is performed in astate of contact of at least one surface with the aqueous solution,since the excess moisture amount is lower when compared with totalimmersion, subsequent treatment steps may be performed in less time.However, since crack formation tends to occur when the amount of aqueoussolution is excessively low, it is preferred that an amount of moistureis used that is equivalent to or greater than the weight of the firedbody.

It is noted that when pores are formed in an inner portion of the oxidefired body, even after the regeneration step, there will be residualpores in an inner portion of the LDH solidified body.

5. Dehydration Step

Next, an inorganic solid electrolyte 16 is obtained by removal of excessmoisture from the LDH solidified body. The step of removal of excessmoisture is preferably performed at less than or equal to 300 degreesC., and in an environment of greater than or equal to 25% of theestimated relative humidity at the maximum temperature of the removalstep. When performing dehydration at a temperature that is higher thanroom temperature, it is preferred to re-enclose in the sealed vesselused in the LDH regeneration step in order to prevent rapid vaporizationof the moisture from the LDH solidified body. The preferred temperatureat that time is 50 degrees C. to 250 degrees C., with a temperature of100 degrees C. to 200 degrees C. being more preferred. Furthermore, thepreferred relative humidity during dehydration is 25% to 70%, with 40%to 60% being more preferred. Dehydration may be performed at roomtemperature and the relative humidity at that time may be configured tofall within a range of 40% to 70% of a normal indoor environment.

When the inorganic solid electrolyte 16 is imparted with a fuelpermeation function by means of the first method described above (methodof providing through holes), the through holes are directly formed inthe inorganic solid electrolyte 16 after removal of excess moisture. Thethrough holes may be formed by using a laser to open holes in thethickness direction of the inorganic solid electrolyte 16. The innerdiameter of the through holes may be adjusted by varying the output ofthe laser or the irradiation time.

Examples

Although the examples of the present invention will be described below,the present invention is not thereby limited to the following examples.

Preparation Solid Alkaline Fuel Cell 10

(1) Preparation of Inorganic Solid Electrolyte 16

Firstly a starting material powder was prepared as a hydrotalcite powder(DHT-4H manufactured by Kyowa Chemical Industry Co., Ltd.) that is acommercially-available layered double hydroxide. The composition of thestarting material powder was Mg²⁺ _(0.68)Al³⁺ _(0.32)(OH)₂CO₃ ²⁻_(0.16).mH₂O. A disc-shaped die having a diameter of 20 mm was filledwith the starting material powder and uniaxial press molding wasperformed at a molding pressure of 500 kgf/cm² to thereby obtain a greenbody exhibiting 53% relative density, 20 mm diameter and an approximatethickness of 0.8 mm. The measurement of the relative density wasperformed on a green body after storage for 24 hours at room temperaturein a relative humidity of less than or equal to 20%.

Next, the resulting green body was fired in an alumina sheath.Temperature increase during firing had a rate of less than or equal to100 degrees C./h in order to prevent fracture of the green body causedby emission of moisture or carbon dioxide due to a sharp temperatureincrease. Then after reaching a maximum temperature of 750 degrees C.,it was retained for 5 hours and allowed to cool. The total firing timefrom temperature increase to temperature decrease (less than or equal to100 degrees C.) was 62 hours.

Next, the fired body was placed in a sealed vessel manufactured fromTeflon (Registered Trademark) provided with an outer jacket of stainlesssteel and sealed together with ion exchange water in air. Afterperforming hydrothermal synthesis on the fired body under regenerationconditions for a retention period of 5 hours at 100 degrees C.,polishing was performed to a thickness of 0.3 mm and moisture on thefired body surface was wiped using filtration paper. In this manner, theinorganic solid electrolyte 16 was formed by natural dehydration(drying) indoors of the resulting fired body at 25 degrees C. and arelative humidity of 50%.

Next, in Examples 1 to 9 and Comparative Examples 2 and 3, through holesoriented in a thickness direction were formed in the inorganic solidelectrolyte 16 by use of a laser. In that configuration, the fuel amountthat permeates the inorganic solid electrolyte 16 was adjusted bycontrolling the laser output and the irradiation time to thereby adjustthe inner diameter of the through hole. In this manner, as shown inTable 1, it was possible to vary the production amount in each sample ofcarbon dioxide per unit surface area in the cathode-side surface 16S ofthe inorganic solid electrolyte 16.

2. Preparation of Cathode 12 and Anode 14

A platinum-supporting carbon having a Pt support amount of 50 wt %(TEC10E50E manufactured by Tanaka Kikinzoku Kogyo) (referred to below as“Pt/C”) and PVDF powder as a binder (referred to below as “PVDF binder”)were prepared. A cathode paste was prepared by mixing Pt/C, the PVDFbinder and water into a paste so that the weight ratio of (Pt/Ccatalyst):(PVDF binder):(water) coincided with a ratio of 9 wt %:0.9 wt%:90 wt %.

In addition, a platinum/ruthenium-supporting carbon having a Pt—Rusupport amount of 54 wt % (TEC61E54 manufactured by Tanaka KikinzokuKogyo) (referred to below as “Pt—Ru/C”) and PVDF binder were prepared.An anode paste was prepared by mixing Pt—Ru/C, the PVDF binder and waterinto a paste so that the weight ratio of (Pt—Ru/C catalyst):(PVDFbinder):(water) coincided with a ratio of 9 wt %:0.9 wt %:90 wt %.

Next, the cathode paste was printed onto one surface of the inorganicsolid electrolyte 16 and the anode paste was printed onto the othersurface of the inorganic solid electrolyte 16. Then a conjugate of thecathode 12/inorganic solid electrolyte 16/anode 14 was formed by thermaltreatment for 4 hours at a temperature of 180 degrees C. in N₂.

(3) Assembly of Solid Alkaline Fuel Cell 10

FIG. 2 is an exploded perspective view illustrating the solid alkalinefuel cell 10 that was prepared according to an Example.

Firstly the inorganic solid electrolyte 16 was fitted to a circularopening (diameter 20 mm) of an electrolyte fixing jig 218 and fixed byuse of PTFE tape 220 that is provided with a circular opening 220 ahaving a diameter of 19 mm formed a central portion. It is noted thatalthough the cathode 12 and the anode 14 were printed onto both surfacesof the inorganic solid electrolyte 16, FIG. 2 illustrates aconfiguration in which there is separation from the inorganic solidelectrolyte 16.

Next, a gasket 222 (made of PTFE) formed with an opening 222 a, an airsupply member 213 (made of carbon) for supply of humidified air and acurrent collecting plate 226 (made of gold plated copper) were stackedonto the cathode 12 side of the stacked body formed from the anode14/inorganic solid electrolyte 16/cathode 12. The air supply member 213is provided with a passage 213 a for allowing flow of humidified air anda slit (not shown) for supply of the humidified air to the cathode 12.

Next, a gasket 224 (made of PTFE) formed with an opening 224 a, a fuelsupply member 215 (made of carbon) for supply of fuel and a currentcollecting plate 228 (made of gold plated copper) were stacked onto theanode 14 side of the stacked body formed from the anode 14/inorganicsolid electrolyte 16/cathode 12. The fuel supply member 215 is providedwith a passage 215 a for allowing flow of fuel and a slit 215 b forsupply of the fuel to the anode.

The stacked body described above was finished by threadable attachmentof screws 230 that were inserted into screw holes 226 a, 228 a formed inthe four corners of the two current collecting plates 226, 228 tothereby complete the solid alkaline fuel cell 10.

Operational Testing of Solid Alkaline Fuel Cell 10

Firstly the solid alkaline fuel cell 10 was heated to 120 degrees C.

Next, a compressor was used to supply the air supply member 213 with airsuch that suitably humidified air was supplied as dry air at less thanor equal to the dew point of 0 degrees C., and adjusted so that the airutilization ratio at the cathode 12 was 50%. The humidification of theair was executed with a humidifier that performed heat exchange with theoutlet gas for cathode air and that was provided with a heater in anexternal portion. Furthermore, gaseous methanol was supplied to the fuelsupply member 215 and adjusted so that the fuel utilization ratio at theanode 14 was 50%. The fuel pressure supplied to the anode 14 wasadjusted to be greater than the air pressure supplied to the cathode 12by use of a discharge pressure adjustment valve (not shown) that wasmounted onto the respective discharge passages for the air supply member213 and the fuel supply member 215.

Then, while generating power at a rated load (0.3 A/cm²), the output ofthe solid alkaline fuel cell 10 was measured when varying the output ofthe humidifier to ceasing operation (0%), 50% and 70%. The output of thehumidifier is a value that is standardized by taking as 100% a point atwhich the output of the solid alkaline fuel cell 10 is greater than orequal to 90% of a maximum output when the water amount required forpower generation at the rated load is provided and the heater output isincreased. In Table 1, ⊚ denotes an evaluation in which the output ofthe solid alkaline fuel cell 10 is greater than or equal to 95% of themaximum output, ∘ denotes an evaluation in which it is greater than orequal to 90% and less than 95%, Δ denotes an evaluation in which it isgreater than or equal to 80% and less than 90%, and X denotes anevaluation in which it is less than 80%.

The produced amount of carbon dioxide per unit surface area of thecathode-side surface 16S was measured using gas chromatograph equipmentto measure the carbon dioxide amount [μmol] contained in the oxidantthat is totally recovered after passing through the cathode 12. Themeasured carbon dioxide amount was calculated by dividing the measuredcarbon dioxide amount by the surface area of the cathode-side surface16S and the operation time.

TABLE 1 Produced Amount of Carbon Dioxide per Rated Load (300 mA/cm²)Unit Surface Area of Humidifier Humidi- Humidi- Cathode-side SurfaceOperation fier fier (μmol/s · cm²) Stopped 50% 70% Example 1 0.04 X X ΔExample 2 0.15 X Δ ◯ Example 3 0.50 X ◯ ⊚ Example 4 0.60 ◯ ⊚ ⊚ Example 51.0 ◯ ⊚ ⊚ Example 6 1.5 ◯ ⊚ ⊚ Example 7 1.7 ⊚ ⊚ ⊚ Example 8 2.0 ◯ ◯ ◯Example 9 2.5 Δ Δ Δ Comparative N.D. (less than or X X X Example 1 equalto lower limit of detection) Comparative 0.02 X X X Example 2Comparative 3.0 X X X Example 3

As shown by Table 1, in Comparative Example 1 in which through holeswere not formed in the inorganic solid electrolyte 16 and in ComparativeExample 2 in which the produced amount of carbon dioxide per unitsurface area was 0.02 μmol/s·cm², when the humidifier output at a ratedload was less than or equal to 70%, the output of the solid alkalinefuel cell 10 was less than 80% of the maximum output. The reason forthis result is considered to be that water required for electrochemicalreactions in the cathode 12 was insufficient due to the fact that theinorganic solid electrolyte 16 does not enable permeation of asufficient fuel amount to the cathode 12 side. Furthermore, inComparative Example 3 in which the produced amount of carbon dioxide perunit surface area was 3.0 μmol/s·cm², when the humidifier output at arated load falls to less than or equal to 70%, the output of the solidalkaline fuel cell 10 was less than 80% of the maximum output. Thereason for this result is considered to be the adverse effect on thecharacteristics of the inorganic solid electrolyte 16 itself due to thefact that the ratio occupied by through holes relative to the inorganicsolid electrolyte 16 was large.

On the other hand, in Examples 1 to 9 in which the produced amount ofcarbon dioxide per unit surface area was greater than or equal to 0.04μmol/s·cm² and less than or equal to 2.5 μmol/s·cm², even when thehumidifier output at a rated load was reduced to 70%, it was possible tomaintain greater than or equal to 80% of the maximum output.

In particular, in Example 2 in which the produced amount of carbondioxide per unit surface area was 0.15 μmol/s·cm², it was possible tomaintain greater than or equal to 80% of the maximum output even whenthe humidifier output at a rated load was reduced to 50%, and in Example3 in which the produced amount of carbon dioxide per unit surface areawas 0.50 μmol/s·cm², it was possible to maintain greater than or equalto 90% of the maximum output even when the humidifier output at a ratedload was reduced to 50%. Furthermore in Examples 4 to 9 in which theproduced amount of carbon dioxide per unit surface area was greater thanor equal to 0.6 μmol/s·cm², it was possible to maintain greater than orequal to 80% of the maximum output even when operation of the humidifierat a rated load was stopped.

In Example 8 in which the produced amount of carbon dioxide per unitsurface area was 2.0 μmol/s·cm², the maximum output was improved incomparison to Example 9 in which the produced amount of carbon dioxideper unit surface area was 2.5 μmol/s·cm². Furthermore, in Example 7 inwhich the produced amount of carbon dioxide per unit surface area was1.7 μmol/s·cm², the maximum output was further improved in comparison toExample 8 in which the produced amount of carbon dioxide per unitsurface area was 2.0 μmol/s·cm².

REFERENCE SIGNS LIST

1. A solid alkaline fuel cell comprising; a cathode supplied with anoxidant which contains oxygen, an anode supplied with a fuel whichcontains hydrogen atoms, and an inorganic solid electrolyte disposedbetween the anode and the cathode, the inorganic solid electrolyteexhibiting hydroxide ion conductivity, wherein the inorganic solidelectrolyte enables permeation of a fuel in an amount that producescarbon dioxide at the cathode of greater than or equal to 0.04μmol/s·cm² and less than or equal to 2.5 μmol/s·cm² per unit surfacearea of a cathode-side surface.
 2. The solid alkaline fuel cellaccording to claim 1, wherein the organic solid electrolyte enablespermeation of fuel in an amount that produces carbon dioxide at thecathode of greater than or equal to 0.15 μmol/s·cm² per unit surfacearea of the cathode-side surface.
 3. The solid alkaline fuel cellaccording to claim 1, wherein the organic solid electrolyte enablespermeation of fuel in an amount that produces carbon dioxide at thecathode of greater than or equal to 0.6 μmol/s·cm² per unit surface areaof the cathode-side surface.
 4. The solid alkaline fuel cell accordingto claim 1, wherein the organic solid electrolyte enables permeation offuel in an amount that produces carbon dioxide at the cathode of lessthan or equal to 1.7 μmol/s·cm² per unit surface area of thecathode-side surface.